Subsurface fluxes dominate the dissolved nitrogen losses from rice paddies in central China

Nitrogen losses in rice paddy fields pose a high risk to surrounding aquatic environment. While the surface N losses have garnered considerable attention in recent years, subsurface flux, including vertical leaching and near-ditch lateral seepage, has barely been quantified. To understand how it impacts the N run off, we collected three-year high-frequency and systematic field observations and adopted a two-dimensional quantitative model coupling water, nitrogen dynamics and crop growth. We quantified the magnitude and dynamics of subsurface N loss during rice growing season. Our results showed that subsurface N loss, reaching 6.2 kg N ha−1 and 4% of total applied N in rice growing seasons, was 2-fold larger than surface N runoff, in which lateral seepage contributed 33%. The driving factors include hydraulic gradients brought by ponded water depth and ditch water level. Based on our study, optimized irrigation management and conservation tillage were priority in mitigating subsurface N loss, with 41% and 32% reduction potential, respectively, and improved ditch-pond system combined with irrigation-drainage management could also be an effective solution. Together, these findings highlight an important role of subsurface flux including lateral seepage, which could be instrumental for future water-quality improvements in rice paddy-ditch landscape.